As integrated circuits have become more complex, the individual devices, such as field effect transistors, forming the integrated circuits have become smaller and more closely spaced to each other. Simple shrinkage of device dimensions was not alone sufficient to permit the increased complexity of the circuits; new processing technologies and innovative devices were also required.
An example will illustrate this point. The source and drain regions of a field effect transistor must be separately electrically contacted. This is frequently done by depositing a dielectric layer over the transistor, patterning the dielectric layer to form windows which expose portions of the source/drain regions, and then depositing a metal in the window. However, to minimize the substrate area used by the device, the source and drain regions should be small. Short channel lengths, determined by the gate width, impose a minimum separation on the windows. That is, the windows must be relatively small and close to each other, but misalignment of the windows with respect to the source/drain regions must not result in a window exposing portions of both a source and a drain region.
An innovative design which decreases the alignment accuracy required for the windows is described in U.S. Pat. Nos. 4,844,776 and 4,922,311 issued to K.-H. Lee, C.-Y. Lu and D. Yaney. These patents describe both a device and a method for making the device which is termed a folded extended window field effect transistor and is commonly referred to by the acronym FEWMOS. In an exemplary embodiment, a layer of a conducting material, such as TiN, is blanket deposited after transistor elements, including an insulating layer on top of the gate electrode, are formed. Of course, WSi.sub.2 could also be used. The conducting material is patterned to form window pads which cover at least portions of the source/drain regions. The window pads may be larger than the source/drain regions provided that they do not contact each other on top of the gate electrode; they may also extend onto the field oxide regions adjacent the source/drain regions. Improved tolerance for window misalignment is obtained because the windows must expose portions of the window pads which are larger than the source/drain regions. Additionally, the window pads may act as etch stop layers thereby preventing etching into the source/drain regions when the windows in the dielectric are etched.
An important transistor design parameter for transistor fabrication is the source/drain region surface area because the junction capacitance is proportional to this area. The junction capacitance is an important parameter in determining device operating frequency. A transistor which uses window pads and thereby has the desirable attributes of FEWMOS and also minimizes source/drain area and thus, junction capacitance, is desirable.